The field of the invention is optical instruments and the invention relates more particularly to refractometers.
It is often important to know whether a diet soda ordered at a restaurant is in fact a diet soda or instead is a sugar-sweetened beverage. This fact can be important to diabetics and other persons who are concerned about possible ill health effects from artificially sweetened beverages. Furthermore, dieters are concerned about the caloric content of sugar-sweetened beverages.
Commercially available refractometers are quite costly but have been known and are in common use by vintners and fruit growers to determine the sugar content of the fruit. This determines whether or not the fruit is ready for harvest. They are also used by beverage makers for quality control.
Typical refractometers have, in addition to their cost, the ability to determine quantitatively the sugar level in a sample. One such refractometer is shown in U.S. Pat. No. 1,522,639. A folding casing is pivoted over a pair of prisms A1 and A2. Internal optics provide an adjustment. The device is adjustable by turning knob G1 and a line of light is visible through lens D2 which permits the user to determine with accuracy the refractive index of the liquid to be tested. Needless to say, the device is expensive.
A somewhat less expensive refractometer is shown in U.S. Pat. No. 2,383,347 where a prism has three exit faces each of which has an objective lens 15, 16 and 17, to project the light beams passing through the three exit faces of prism 3. The refractometer is capable of determining the refractive index of a wide range of liquid samples.
A refractometer with an improved line of demarcation between the light field and the dark field is shown in U.S. Pat. No. 2,972,926. This patent also shows a refractometer illuminated by a light bulb. Another illuminator is shown in a refractometer in U.S. Pat. No. 4,890,916.
Because the refractive index of a sample varies with temperature, refractometers are often equipped with temperature compensating mechanisms. They have adjustable focus eye pieces to provide a sharp image of the scale Commercial devices are fairly bulky and generally cost several hundred dollars.
The sugar-sweetened versions of most soft drinks contain 12% sugar by weight as they come from the bottle or from a properly adjusted drink fountain. These soft drinks vary in sugar level to as low as 6% after dilution by melted ice. At this concentration, the beverage is so watery most people find it undrinkable. The diet versions contain very nearly 0% sugar by weight and in fact, are virtually indistinguishable by refractometer measurement from tap water.
The inventor wished to provide a device which could be easily afforded by persons needing to know whether a drink is sugar-sweetened or not. People with phenyl ketonuria (PKU) are unable to properly metabolize phenylalanine and must avoid aspartame, a common artificial sweetener. Other people experience nervous system disturbances when they drink beverages containing aspartame and may wish to avoid diet soft drinks. Many people are wary of the possible long term health effect of consuming artificial sweeteners and may want to avoid them to reduce their risk of cancer and other illnesses. Dieters also are concerned about their caloric intake.
Various alternative test methods are available for determining the presence of sugar. One such test is a urine test tape which when contacted with a sugar-sweetened substance changes color. There are two disadvantages of this test. First, it is relatively expensive. Secondly, the strips have a relatively short life span since the color indicator is somewhat unstable. Also, one ends up with a wet strip which must be disposed of.